1. Field of the Invention
The present invention relates to a glass forming mold, particularly to a glass forming mold to be used for forming a glass product such as a panel or funnel for a cathode ray tube for a television.
2. Discussion of Background
A cathode ray tube for a television is prepared by producing a front (image screen) panel and a rear funnel separately, coating a phosphor on the inner surface of the panel, attaching a shadow mask, electrodes, an electron gun, etc., followed by vacuum welding by means of a frit. Especially for a panel, an extremely strict quality control is required with respect to the surface precision of the inner surface of the panel and the state of e.g. surface irregularities, as it serves as an image screen. The panel and the funnel are formed by pressing a high temperature molten glass at a level of about 1,000° C. against a mold. Accordingly, the mold to be used for forming, is required to have properties such as mechanical strength, heat resistance, chemical stability against the high temperature molten glass, etc.
Stainless steel is known as a base material for such a mold, since it is inexpensive and capable of being processed at a high precision and into a complicated shape. It is also known to use a mold having a chromium coating film formed on the surface of stainless steel, in order to impart oxidation resistance.
However, the surface of a chromium coating film on the mold surface is gradually oxidized to form a chromium oxide film, if a high temperature molten glass which contains a large amount of oxidative substances (such as silicon oxide, sodium oxide, potassium oxide, strontium oxide, barium oxide and lead oxide), like a glass for a cathode ray tube, is formed for a long period of time. Consequently, the surface roughness, surface precision, release property from the product, etc. of the mold surface, tend to deteriorate, and high temperature corrosion of the surface of the chromium coating film will proceed. If this corrosion further proceeds, the quality of the glass product will deteriorate, and finally, it will be necessary to change the mold itself. After the change of the mold, it takes some time until glass forming will be stabilized, whereby the productivity decreases remarkably. Accordingly, it contributes substantially to the improvement of the productivity to control the high temperature corrosion of the surface of the chromium coating film. As a method for controlling the high temperature corrosion of the surface of the chromium coating film, a method is, for example, known wherein the surface of the chromium coating film is subjected to anodic oxidation to form a corrosion resistant chromium oxide film on the surface. However, the oxide film obtained by such anodic oxidation has had a drawback that the oxide film itself tends to be thick and peeled as it is used for forming a high temperature molten glass for a long period of time. Further, a method is also employed wherein heat resistant carbon is coated on the surface of the chromium coating film on the mold surface to suppress the corrosion of the chromium coating film, but there has been a drawback such that the effect to suppress the corrosion is rather limited to a short period of time, and the coated portion is susceptible to peeling.